Additives for lubricating agents used in the lamination of lithium sheets into thin films

ABSTRACT

Additives are represented by the following general formula: 
     
         L-Y-S 
    
     in which L designates a hydrocarbon radical which serves as lubricating segment; S designates an oligomer segment which serves as solvating segment of metallic salts and Y designates a chemical bond which joins the hydrocarbon radical and the oligomer segment. With these additives there is no more need to subsequently wash the surface of laminated lithium.

BACKGROUND OF INVENTION

a) Field of the Invention

The invention concerns additives which may be used as laminationlubricating agents or are part of lamination lubricating agents. Theinvention also concerns compositions including these additives and whichmay be used in the lamination of a sheet such as lithium in order toobtain thin films, which may be used as such in the production ofpolymer electrolyte electrochemical cells. In addition, the inventionconcerns the use of the additives per se or compositions containing sameto provide, by lamination, films of alkali metals or alloys thereofwhich may be used as anodes in electrochemical cells preferably withpolymer electrolytes. The invention also concerns a process oflamination utilizing these additives or compositions containing same aslamination lubricating agents.

b) Description of Prior Art

The production of thin films of lithium having thicknesses lower that 75micrometers and in the form of wide bands, for example 5 centimeters ormore and in lengths of many tens of meters, by means of rapid andreliable processes, faces important technical difficulties which areattributable to the extreme physical and chemical properties of thismetal: chemical reactivity, malleability, rapid self-welding by simplecontact and strong adhesion on most solid materials, for example theusual metals.

This difficulty is confirmed by the difficulty of obtaining fromsuppliers of specialty metals and chemical products, thin lithium films40 micrometers (μm) thick and less, of sufficient surface and length,having an adequate surface finish and chemical property to be used inlithium cells.

Presently, cold extrusion is used for the continuous production ofsheets 75 μm and more. These thicknesses are generally adapted to theproduction of lithium cells utilizing liquid electrolytes. For lowerthicknesses, the films obtained by extrusion are thereafter laminatedbetween rollers made of hard materials. These processes have beendescribed and are commercially used for the production of limitedquantities of sheets of 30-75 microns. Reference will particularly bemade to U.S. Pat. No. 3,721,113, inventor Hovsepian and dated Mar. 20,1973. Many successive passes, according to the present state of the art,are required to give films 40-30 μm.

Other alternative processes have been described to give ultra-thinsheets, which are used for example in the production of polymerelectrolyte cells in the form of thin films. This is the case forexample of a lamination process between steel rollers which areprotected by films of hard plastic which are non reactive towardslithium, such as described in U.S. Pat. No. 3,721,113, or of processesbased on the coating of molten lithium on a metallic of plastic support,described in U.S. Pat. No. 4,824,746, inventors Andre Belanger, et al,dated Apr. 25, 1989.

The difficulty in achieving the lamination of lithium to thicknesseswhich vary between 40 and 5 microns for the production of polymerelectrolyte cells is mainly due to the reactivity and the adhesion ofthe laminated metal with the materials with which it is in contact:lamination rollers, protection plastic films, lamination additives, aswell as to the bad mechanical properties of thin sheets. For example, afilm of lithium 20 μm thick and 10 cm wide breaks under a drawingtension higher than 579.13 KPa which does not permit to pull on the filmwhich exits from the laminating machine or to release it from thelamination rollers if lithium adheres somewhat thereto.

An approach which is normally used for the extensive lamination orcalandering of hard metals, such as iron and nickel, is based on the useof liquid lamination additives consisting of organic solvents which maycontain greases or lubricating agents. Examples include fatty acids orderivatives thereof such as for example lauric or stearic acids andalcohols, for example the compounds known under the trade marks EPAL1012 of Ethyl Corporation U.S.A., which are mixtures of primary linearC₁₀ --C₁₂ alcohols.

For lithium and particularly for lithium intended for electrochemicalcells, the use of such additives involves two major difficulties:

1) the chemical reactivity of lithium which is in contact with solventsor lubricating agents including reactive organic functions, such asorganic acids and alcohols. These functions react at the surface oflithium during and after lamination and create passivation films at thesurface of the metal. This is harmful for a good operation ofelectrochemical cells especially when the latter are intended to berechargeable;

2) the difficulty of removing the lubricating agents or greases whichare in contact with lithium after lamination. This is the case, forexample, when lubricating agents which mostly consist of hydrocarbonchains are selected, because they are nearly not reactive with lithium.These compounds constitute electrical insulating materials which areharmful to the good operation of lithium electrodes made with thesesheets. Such lubricating agents are not very soluble in polymerelectrolytes and should therefore be removed from the surface of lithiumby washing after lamination. In addition to the fact that the washing ofthe surface of lithium is a delicate and costly operation, it will benoted that this operation inevitably contributes to contaminate thesurface of lithium, in spite of all the care which may be used tocontrol the quality of the surface of the metal. The latter reactsindeed irreversibly with all the impurities, including water, which arepresent in the washing solvents, or resulting from accidentalcontaminations.

It can be shown that the lithium obtained after a process of laminationwith an additive followed by a subsequent washing is generally morecontaminated at the surface than a lithium which is laminated withoutadditive. This phenomenon may be observed with optical means, includinga simple visual inspection or by a control of the impedance of theelectrochemical batteries produced with polymer electrolytes. On theother hand, lamination without solvent and without lubricating agentmeans low production speeds and a tendency of the fresh lithium to stickto the rollers or the protection films of the rollers; moreover, manyconsecutive laminations are therefore required to reach thicknesses ofthe sheet lower than 40 micrometers.

SUMMARY OF INVENTION

It is an object of the present invention to solve the problem oflamination or calandering of lithium films, to thicknesses between 40and 5 μm, which can be directly used in lithium batteries made with thinfilms, for example polymer electrolyte batteries.

It is also an object of the invention to propose lubricating additiveswhich are chemically compatible with lithium and which may be used in aprocess of lamination which does not require a subsequent washing of thesurface of laminated lithium.

Another object of the invention resides in a composition consisting of alamination lubricating agent including an appropriate solvent as well asan additive having two functions.

Another object of the invention resides in an improvement of the processof lamination of lithium in the presence of an improved lubricatingagent.

Another object of the invention is to propose lamination lubricatingadditives enabling to produce in a single pass, extremely thin lithium,for example a thickness lower than 10 μm, at appreciable speed which maybe up to 50 m/min., and even more, and with an excellent control of thesurface properties: uniform surface profile and low impedance of thepassivation layer when the sheets thus produced are used in anelectrochemical cell.

Another object of the invention consists in the provision of alamination lubricating agent including an additive and solvents, inwhich the latter are selected for their chemical compatibility with alithium which is intended for an electrochemical cell.

As used in the present description and in the appended claims, chemicalcompatibility of solvent or of an additive toward lithium of anelectrochemical generator means the absence of chemical reaction withlithium or also, a limited chemical reaction leading to the formation ofa passivation film which is not harmful to electrochemical exchanges, atthe interface lithium/electrolyte of said cell.

Another object of the invention resides in the chemical formulation of alubricating agent for use in lamination which is not volatile and isselected so that it may be kept at the surface of lithium afterlamination and this without harming the good operation of the sheet oflithium (anode), when the latter is used as such in an electrochemicalcell, i.e. without any previous washing step.

Another object of the invention resides in an improved process oflamination utilizing the additives according to the present invention.

The invention is based on the choice of a lubricating chemical compoundof high molecular weight including at least two segments of differentchemical nature: a chain or a chain segment having a lubricatingfunction (L) as made, for example, of a hydrocarbon chain including atleast 8 carbon atoms associated with a solvating chain (S), capable ofionically dissociating at least in part a metallic salt, for example oflithium, such as a chain segment of ethylene polyoxide. The solvatingsegment present in the lubricating additive is selected so as to conferan ionic conductivity to the lubricating additive.

A preferred but non limiting manner of inducing ionic conductivity inthe lubricating additive is obtained when the laminated lithium iscontacted with the electrolyte (solvating polymer+lithium salt) of thecell. The salt present in the electrolyte is then diffused in thesolvating part of the additive and locally constitutes a complexconductor (solvating chain+salt).

The lubricating agent according to the invention comprises at least onesequence:

    L-Y-S

where:

L designates a hydrocarbon radical, such as alkyl, alkylene, linear orcyclic or aryl-alkyl, saturated or non saturated, preferably containingmore than 8 carbon atoms used as a lubricating segment which iscompatible with lithium;

S designates an oligomer segment including heteroatoms such as O or N,and capable of solvating salts, for example salts of lithium andensuring an electrolytic conductivity;

Y designates a chemical bond or a chemical group which is at leastdivalent joining the chains or chain segments L and S.

The solvating cell segment S may be joined to a terminal group C toconstitute the sequence L-Y-S-C, C then being selected for its lowreactivity with lithium.

C may for example designate a group Y'-L', which is identical ordifferent from group Y-L, an alkyl radical, an alkyl-aryl radical, ofvalence equal to or higher than 1. According to a variant, C is apolymerisable group which can be incorporated to at least one of therepetitive units which constitute the polymer electrolyte of anelectrochemical cell. According to another variant, C includes aionophoric group which is somewhat dissociable and is capable ofinducing an intrinsic ionic conductivity in the additive.

Examples of polymeric solvating chains are given in the followingpatents: U.S. Pat. No. 4,303,748, inventors Michel Armand, et al, datedDec. 1, 1981, and U.S. Pat. No. 4,578,326, inventors Michel Armand, etal, dated Mar. 25, 1986. Chains bases on ethylene oxide - CH₂ --CH₂--O!_(n) -, propylene oxide - CH₂ --CH₂ (CH₃)--O!_(n) -or onpoly-(N-methyl-ethyleneimine) - CH₂ --CH₂ --N(CH₃)!_(n) or theircombinations are generally preferred, but other solvating functions mayalso be used as long as they may induce an ionic conductivity in thelubricating additive.

In the case where the hydrocarbon segment originates from a fatty acid,the bond Y preferably consists of ester (L)-CO-O-(S) or ether (L)-O-(S)groups. Y may also represent amine or amide groups.

According to a preferred embodiment of the invention, the segment maycorrespond to the hydrocarbon chain of a fatty acid including at least 8and preferably from 10 to 30 carbon atoms. For example, L may consist ofa hydrocarbon chain of a fatty acid such as stearic acid and Y may thenbe a chemical bond of the ester or ether type, or may represent acarboxylate group which originates from a fatty acid ester.

According to another preferred embodiment of the invention, the segmentS may consist of polyethers or polyamines of molecular weights 150.

According to another preferred embodiment of the invention, the terminalgroup C may also include a chemical function capable of covalentlyfixing a metallic salt, for example a lithium salt.

According to another preferred embodiment of the invention, the chemicalbond C may include a lithium salt which is chemically grafted by theanion or by means of one or more unsaturations.

The invention also resides in a lithium film covered with a thin layerof the additive defined above, the thickness of the film being between 5and 50 microns.

Another aspect of the invention concerns a lithium based anode preparedfrom a sheet of lithium covered with a thin layer of the additivedefined above, the thickness of the anode being between 5 and 50 μm,which is in direct contact with a sheet including carbon or metalscapable of chemically forming an alloy of lithium or an intercalatingcompound of lithium.

The invention also concerns a polymer electrolyte electrochemical cellincluding a lithium anode which is prepared as indicated above, in whicha free lithium salt is present in the electrolyte so as to form, bydiffusion, a complex electrolyte conductor with the chain S of theadditive, and the latter may be soluble in the electrolyte.

According to another embodiment of the invention, there is provided theuse of an additive or a composition as defined above for producing filmsof alkali metals or alloys thereof by lamination, which may be used asanodes in polymer electrolyte electrochemical cells.

The invention finally concerns a process of lamination which is intendedto give thin films of alkali metals or alloys thereof, from a sheet ofsaid metals or alloys thereof wherein the sheet is passed betweenworking rollers with a laminating lubricating agent to laminate thesheet into thin films, characterized in that the lubricating agentincludes an additive or a composition as defined above.

A particularly interesting additive is a polyoxyethylene distearatewhose solvating segment corresponds to a molecular weight between about150 and 4000.

The compositions according to the invention preferably contain 0.01 to10% by weight of additive, more specifically about 0.2%. With respect tothe solvent, it may be selected among saturated or partially saturatedlinear, cyclic or aromatic hydrocarbons, for example heptane, benzene,toluene, cyclohexane or a mixture thereof. It may also be selected amongaprotic solvents which are compatible with lithium.

A particularly advantageous formulation consists in using a family ofcompounds of the type: L-Y-S-Y-L based on diesters of fatty acids, forexample polyether glycol stearates, such as the compounds, CH₃ --(CH₂)₁₆--COO--(CH₂ --CH₂ --O)_(n) --OOC(CH₂)₁₆ --CH₃ where n preferably variesbetween 3 and 100. Compounds including polyether segments of molecularweight equal to 200, 400 and 600 are commercially available fromPolyscience, preferably POE 400 Aldrich No. 30541-3.

The stearate segments have excellent lubricating properties and theirhydrocarbon chains are inert towards lithium; in this case, the bond Yis ensured by the carboxylic group of the starting fatty acid. Theterminal group C then consists of a segment Y'-L' identical to L-Y.

It has been observed that a central polyether chain, of low molecularweight, distearate POE 200, is sufficient to give to the lubricatingcompounds an ionic conductivity of the order to 1×10⁻⁵⁻ S.cm at ambienttemperature when a lithium salt such as Li(CF₃ SO₂)₂ NLi is, added in aratio such that the ratio O/Li is 30/1. This value is amply sufficientto ensure ionic exchanges at the lithium/electrolyte interface of anelectrochemical cell taking into account the small thickness of theresidual deposit of the lubricating agent after lamination.

These preferred formulations are given by way of example of possibleembodiments of the invention. Other lubricating and solvating functionsL and S may be used as well as other bonds Y. By way of non limitingexample, reference may be made to the following articles which deal withtypes of solvating chains:

Polymer Electrolytes review-1, J. R. MacCallum & C. A. Vincent eds.Elsevier Applied Science London (1987);

Polymer Electrolyte, review 2, J. R. MacCallum & C. A. Vincent eds.Elsevier Applied Science London (1989);

Solid Polymer Electrolytes, F. M. Gray VCH Publisher New-York, Weinheim(1991); as well as

Surface Active Ethylene Oxide Adducts, by V. Schoenfeldt-Permagon Press,(1966).

The preparation of the additives according to the present invention iswell known to one skilled in the art and needs no detailed discussion inthe present context. It is sufficient to mention that any skilledchemist would have no problem to synthesize the desired additive oncethe solvating and lubricating chains are established and the choice ofthe chemical bond which is intended to be used has been made.

During lamination, it is generally preferable to dilute the lubricantingagents according to the invention in one or more solvents which arecompatible with lithium and which are preferably linear, saturated orpartially unsaturated, or cyclic aromatic hydrocarbons such as heptane,benzene, toluene, cyclohexane or any other pre-dehydrated aproticorganic solvent or a mixture thereof. This dilution enables to reduce toa minimum the required quantity of lubricant and to obtain optimumqualities of lithium for use in an electrochemical cell. These solventsare previously dehydrated, for example on a molecular sieve, to lowerthe water content below 100 ppm. The concentrations of additives mayvary up to about 10% by weight for example between 0.01 and 10% byweight, preferably 0.2% by weight. The addition of the lubricantingagent in solution is carried out in a controlled manner immediatelybefore lamination between rollers. The laminated film is dried by acontinuous operation with dry air immediately at the outlet of therollers and is thereafter wound with or without a separator film ofinert plastic, preferably of propylene or polyethylene.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood by the annexed drawings given byway of illustration but without limitation, in which:

the single FIGURE is a schematic illustration of a laminating operationutilizing an additive according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

It will be seen that a lithium sheet 1 having a thickness of about 250micrometers mounted on an unwinding drum (not illustrated) is passedbetween two working rollers 3 and 5 made of polyacetal. A pressure isapplied on the two rollers in the direction indicated by arrows 7 and 9,which is sufficient to reduce the thickness of the sheet by about 90%.At the inlet of the sheet between the laminating rollers, a laminationlubricant 11 is poured, for example toluene, from a pouring spout 13.

At the outlet of the two lamination rolls, the sheet of lithium isconverted into a film 15 whose thickness is about 25 micrometers. On theother hand, it will be realized that the film 15 remains in adhesion onthe surface of the roll 3 from the meeting point 17 between the tworollers 3 and 5, up to a given limit point 19 on the circumference ofthe roller 3 forming an angle α of about 90° with the meeting point 17.

Film 15 is thereafter wound onto a winding drum (not illustrated) withsufficient tension, determined empirically for, on the one hand, causingthe film 15 to move from point 19 to be gradually brought to point 21where the operation is continued without any other change.

Normally, at point 21, the angle β formed will be about 45°, it beingunderstood that this angle may vary depending on circumstances and thedesired properties of the film of lithium 15.

An advantageous way to carry out the invention is described in thepatent application filed simultaneously herewith and directed to aprocess of lamination in a single pass, between two rollers of hardplastic. This procedure which is preferably carried out in a single passrelies on the control of the adhesion on one of the plastic rollers soas to pull the lithium according to a preferred angle and to control itsinherent flatness.

Other processes of lamination utilizing metallic rollers are alsopossible while using these additives. Thus, the metallic rollers couldbe pre-coated with lubricant so as to minimize the adhesiveness.However, the concentration as well as the chemical nature of theadditives according to the present invention should be adjusted as afunction of the intended production speeds.

These additives are also applicable to the lamination of lithiumenriched alloys such as lithium-boron or lithium-magnesium alloys oralso to the lamination of other alkali metals, for example sodium andsodium-lead alloys.

The process, the compositions and the additives according to the presentinvention are also applicable to the preparation of lithium anodes whichare used in liquid electrolyte cells as long as the residual film isconductive or soluble in the electrolyte. Similarly, the process and theadditives according to the present invention may be used to chemicallyprepared anodes of lithium alloys or based on carbon-lithium.

Advantageously but without limitation, it is possible to use as additiveaccording to the invention the following chemical products:

Polyoxyethylene distearates in which the solvating segment has amolecular weight (mol. st.) equivalent to 200, 400 and 600, for exampledistearate 400 of Aldrich No. 30541-3.

Non-ionic surfactants: BRIJ® of ICI America available at Aldrich undercatalogue Nos:

85,836-6 BRIJ® 35

23,599-7 BRIJ® 58

23,600-4 BRIJ® 78

23,865-1 IGEPAL® CO-720

23,869-4 IGEPAL® DM-970

Other possible products are illustrated by:

Distearates (dilaurates, dipalmitates, dioleates)

of POE (200, 4000 mol. wt.)

of polypropylene glycol (725, 1000, 2000, 3000)

of PLURONIC® (OE-OP blocks)

of polyoxytetramethylene (poly THF) (650, 1000, 2000).

Dihexadecyl ethers of POE (200-4000 mol. weight).

Dicholesteryl carbonates of POE (200-4000).

Tristearates (laurates, palmitates, oleates) of POE, triol (200, 4000)(DKS).

Monostearates (laurates, palmitates, oleates)

of BRIJ (35, 58, 78)

of IGEPAL (CO-720, DM-970).

The polymethacrylates of oligo-oxyethylene-monolaurylether.

It is often preferable to use solvents which are compatible with lithiumfor diluting the lubricating additive. The latter are preferably linearhydrocarbons. The concentrations of the additives may then vary between10 to 20% P/P and less than 0.05% P/P.

The lithium produced by utilizing the additives according to the presentinvention may be used as such in polymer electrolyte cells. Canadianpatent application No. 2,068,290-6 filed on May 8, 1992 describes oneway of producing a complete cell and various ways of establishingelectrical contacts on the lithium sheet. In these cases, the laminationadditive will be made electrolytically conductive by the diffusion ofthe salt of lithium from the film of electrolyte of the cell.

In certain cases, the residual layer remaining after lubrication may bemore or less dissolved or dispersed in the electrolyte, for example whenthe latter is of low molecular weight or comprises liquid aproticsolvents.

Other characteristics and advantages of the present invention willappear from the description which follow of embodiments given by way ofillustration but without limitation.

EXAMPLE 1

In this example, the determining effect of a preferred additiveaccording to the invention on a lamination carried out during acontinuous operation and in a single pass to give a lithium film lessthan 30 micrometers (μ), is established. The device used is the onedescribed in FIG. 1 and the lamination is carried out in an anhydrideatmosphere containing less than 1% relative humidity. The rollers aremade of polyacetal and have a diameter of 20 mm; the starting lithiumconsists of an extruded sheet 250 micrometers (μ) thick. The solventsand the additive, if needed, are previously dehydrated on a molecularsieve in order to give a water concentration lower than 10 ppm.

As a first step, an attempt is made for laminating in a continuousoperation a sheet of lithium 57 mm wide and to reduce its thickness in asingle pass to 25μ. When no lubricating liquid is used during thelamination, lithium immediately adheres to the rollers and the processdoes not operate properly; with the addition of hexane, it is impossibleto achieve lamination unless the rate of reduction of the thickness ofthe sheet is considerably reduced. At the best, we managed to obtain alithium 90μ in a single pass in which the inherent flatness of the filmis extremely bad. Therefore, hexane, as used in the prior art, does notpossess sufficient lubricating properties to be used alone in acontinuous process in a single pass to give a lithium less than 25μ.

When the lamination is carried out with a lubricating liquid consistingof toluene, added at the rate of 8 ml/min. on a extruded sheet 57 mmwide, the lamination of lithium in a continuous operation to 25μ becomespossible and a maximum speed of 5 m/min. is obtained while allowing thelaminated film to adhere to the upper roller up to a quarter of itsheight (angle of 45°), as illustrated in FIG. 1 of the Canadian patentapplication mentioned above. This operation enables to perfectly controlthe tension applied on the free film and gives a lithium of excellentinherent flatness. Lengths of 10 to 20 meters may thus be obtained incontinuous operation. By rapidly changing from toluene to hexane duringthe operation, there is produced an instantaneous rise in the thicknessof the lithium to about 90μ and a lithium of very bad inherent flatnessis obtained.

The interest of the additives according to the invention is establishedby utilizing an extruded lithium 250μ of 143 mm wide. The device of theprevious tests was used with a solution of hexane and toluene in a ratio9:1 containing a distearate POE 200 (mol. weight) at a concentration of0.2% P/P. An excess of lubricating solution is added on the sheet ofextruded lithium at the rate of 6 ml/min. Under these conditions, alithium film 22μ of excellent inherent flatness is obtained in a singlepass at a lamination speed of more than 20 m/min. This process which isstill not optimum additionally enables to produce rolls of laminatedsheets more than 300 meters long in which the thickness is constant atmore or less 2μ. The following productions are highly reproducible fromone test to the other and the rates of losses or interruptions of theprocess are negligible; more important productions are thus possiblestarting from longer rolls of extruded lithium or from a feed to thelaminating rolls, directly from an extruder.

EXAMPLE 2

Lithium 22μ produced by utilizing the additive of example 1 is used asthe anode of a lithium cell operating at 60° C. The visual aspect oflithium is excellent, the lithium is bright without any coloring, andthe surface profile obtained with DEKTAK® (model 3030 of VEECO U.S.A.)fluctuates within 3μ. For this laboratory test, the lithium sheet islightly applied under pressure on a thin nickel sheet to ensure currentcollection. The electrolyte used consists of a polymer electrolyteconsisting of a copolymer of ethylene oxide and methylglycidyl ether anda lithium salt, (CF₃ SO₂)₂ NLi in an oxygen lithium ratio (O/Li) of30/1. The composite cathode consists of vanadium oxide and carbon blackdispersed in the polymer electrolyte and has a capacity of 5 C/cm². Theactive surface of the battery thus constituted is 3.9 cm². The initialimpedance of this battery at 60° C. is 15 Ω, i.e. it is equivalent to orlower than the best lithium obtained commercially. The cyclingproperties of this battery utilizing the lithium of example 1 areexcellent after 100 cycles and the rate of utilization of the batteryremains at least equivalent to similar batteries prepared withcommercial lithium, or about 90% of the initial value stabilized after10 cycles. This example confirms that the presence of the non volatiledistearate of POE which remains at the surface of lithium causes no harmto the good operation of the cell. This result is explained by theelectrolytic conductivity generated by the presence of the POE solvatingsegment of the additive and by the chemical compatibility of the batterywith lithium. In an independent test, the electrolytic conductivity ofthis additive, when the salt content (CF₃ SO₂)₂ NLi is 30/1, is about1×10⁻⁵ S.cm.

EXAMPLE 3

In this example, we have evaluated at a temperature of 25° C. theimpedance of symmetrical batteries Li°/polymer electrolyte/Li° preparedfrom laminated lithium without additive and also when covered with anexcess of various possible lubricating materials.

The quantity of lubricating agent used per surface unit of lithium is0.03 mg/cm². This value corresponds to an excess of lubricating agent ascompared to what is necessary for laminating according to example 1,however the aimed purpose is to amplify and accelerate theelectrochemical effect of various additives. The impedance values aregiven for batteries whose active surface is 3.9 cm². The electrolyte ofexample 1 is also used to prepare batteries which are assembled by hotpressing under vacuum.

For the various material used, the results are the following:

    ______________________________________                                                                   Impedance                                          ______________________________________                                        1)      Distearate of POE 200 (mol. Wt.)                                                                 113Ω                                         2)      Distearate of POE 600 (mol. Wt.)                                                                 113Ω                                         3)      Pure stearic acid  840Ω                                         4)      Pure POE of molecular wt. 500                                                                    139Ω                                         ______________________________________                                    

The values observed confirm the influence of the POE segment on theelectrolytic conductivity of the additives and enable to conclude thatstearic acid often used as lubricating agent for laminating conventionalmetals is incompatible with lithium for use in an electrochemical cell.

EXAMPLE 4

In this example a comparison is made of the effect of various knownlamination additives for their lubricating properties on the efficiencyof lamination of lithium in a single pass from 250μ to about 30μ.

In order to make these comparisons, the lamination is initiated underconditions similar to those of example 1 by utilizing distearate of POE200 as additive. When the lamination is in operation, the composition ofthe solution is modified by replacing the distearate POE with otheradditives. The effect of the addition is immediately noted by followingthe thickness of the laminated lithium film, its inherent flatness andits visual appearance. When the solution containing the distearate isreplaced by a solution of ethyl stearate at a concentration of 0.15%P/P, the thickness of lithium rises suddenly from 40 to 90μ and with aloss of inherent flatness of the laminated lithium.

When changing to a laminating solution based on the lamination lubricantEPAL® 1012 (CO linear alcohol) of Ethyl Corporation, it is noted thatthe thickness of laminated lithium progressively rises beyond 65μ andthat the lithium obtained become sticky at the center of the rollerswhile the sides become irregular (undulations).

When changing to a laminating solution based on POE 5000 in toluene, arapid rise of the thickness of laminated lithium to 90μ with a loss ofinherent flatness is noted.

These tests illustrate the importance of formulations based on stearateswhich act as lubricating agents and include solvating functions such asthose based on POE. These preferred but non-limiting formulations arealso superior to additives based on pure POE in terms of laminationprocess even if the electrolytic conductive properties are in this caseadequate as illustrated in example 3.

EXAMPLE 5

In this example, the POE stearate is replaced by other compounds of theinvention while preserving the other identical conditions. The twocompounds used are: dicholesteryl-carbonate of POE 600 (mol. Wt.) anddipalmitate of POE 4000.

In the two cases, the lamination speed may be maintained and thethickness of the laminated lithium is substantially the same. In thesetwo cases, the inherent flatness of lithium is preserved. These examplesconfirm the generality of the formulations which combine the solvatingand lubricating functions.

EXAMPLE 6

This example describes a compound according to the invention whichincludes the ionophoritic group according to formula L-Y-S-C (where Ccomprises a dissociable metal salt enabling the additive L-Y-S-C to havean intrinsic ionic conductivity). This type of compound is important aslamination additive when the laminated lithium is intended to be usedfor example in cells in which the electrolyte include a salt whose anionis chemically bounded to the polymeric chain. In this case, there is nopossibility for the salt of lithium to diffuse and the lubricatingadditive should include an ionophoretic function to prevent theformation of an insulating deposit at the surface of lithium.

A non-ionic tensio-active agent of the type BRIJ 35®, polyoxyethylene 23lauryl ether C₁₂ H₂₅ (OCH₂ CH₂)₂₃ OH is sulphonated by the followingprocedure: 12 g of BRIJ 35® are dried by azeotropic distillation withbenzene followed by lyophilisation. After addition of 50 ml of THF, theterminal OH groups are metallized with sodium hydride in the presence of5 mg of triphenylmethane. The stoichiometry is determined bycolorimetry, the end of the reaction being indicated by the intense redcolour of the Φ₃ C anion. 1.4 g of 1,4 butane sulphone are then added.After evaporation of the solvent, the sulphonated oligomer is obtainedin the form of powder. 5 g of the product thus formed in suspension in15 ml of acetonitrile are treated with 1 ml of thionyl chloride and 20μl of dimethylformamide. A precipitate of sodium chloride is formed in20 mn. After filtration, the solvent and the excess of SOC₂ areevaporated under reduced pressure. The residue is solubilized in 30 mlof pyridine and added to 1.2 g of the sodium salt ofbis(trifluoromethanesulfonyl)methane. After filtration, the reactionmixture is stirred in the presence of 1 g of lithium phosphate Li₃ PO₄.A new filtration enables to separate a colourless solution which, byconcentration, gives a wax. This material possesses tension activeproperties of lubrication and ionic conduction.

When used under the conditions of Examples 1 and 5, this material alsoenables the lamination of lithium under equivalent conditions. Thisexample is non-limiting and other equivalent materials including a moreor less dissociable ionic function may also be used.

EXAMPLE 7

An extruded sheet of lithium 1 250 micrometers thick and 143 mm wide isused as starting material. The latter is mounted on an unwinding drum,passed between working rollers and the film is rolled onto a windingdrum. A pressure which is sufficient to thin down the film is applied onthe working rollers. These rollers are of polyacetal and have a diameterof 20 mm. The film is mounted on the apparatus between the workingrollers. The pressure on the rollers is increased in order to decreasethe thickness of the film by about 90%. A lubricating agent is added onthe film of lithium at a rate of 6 ml/min. This lubricating agent ismade of a mixture of solvents to which there is added a laminationadditive, which comprises dry hexane and toluene in a ratio of 9:1 and0.2% p/p POE 200 distearate of formula CH₃ (CH₂)₁₆ --COO--(CH₂ --CH₂O)_(n) --OOC(CH₂)₁₆ --CH₃ where n is selected so that the polyethersegment has a molecular weight of 200.

The film is allowed to adhere to ×1/4 of the height of the workingroller so as to perfectly control the tension applied on the latter. Thepressure which is exerted on the rollers is adjusted so as to obtain ina single pass a film of lithium 25 micrometers thick, homogeneous at ±2μm and 300 meters long. It will therefore be seen that it is possible tooperate in a continuous manner without reject.

This additive enables to raise the speed of lamination to 20 m/min andto obtain a thin film of lithium of excellent quality.

We claim:
 1. A sheet of an alkaline metal or an alloy thereof having alayer of non-volatile additive thereon, essentially constituting alubricating agent therefor, said sheet to be used in an electrochemicalcell operating with a polymer electrolyte while said layer remainsbetween said sheet and said polymer electrolyte, wherein said additivehas the formula:

    CH.sub.3 --(CH.sub.2).sub.16 --COO--(CH.sub.2 --CH.sub.2 --O)--OOC (CH.sub.2).sub.16 --CH.sub.3

wherein n ranges from 3-100.
 2. The sheet of claim 1, wherein saidadditive is a distearate of polyoxyethylene, the polyoxyethylene portionof the distearate additive having a weight between 150 and 4,000.